1. Field of the Invention
The present invention relates to a backlight of a liquid crystal display (LCD) device, and more particularly, a backlight driving circuit for preventing damage from a discharge generated by disconnection of a connector between a low-voltage part of a lamp and an inverter.
2. Discussion of the Related Art
With rapid development of information communication fields, the importance displaying desired information has increased dramatically. Recently, cathode ray tubes (CRTs) have been commonly used as display devices in televisions and computer monitors because of their ability to display various colors with high luminance. However, CRTs are relatively large and cannot adequately satisfy present demands for display applications that require reduced weight, portability, low power consumption, and increased screen size and high resolution. Accordingly, flat panel displays have been developed for use as monitors for computers, spacecraft, and aircraft.
Various flat panel displays are in use, for example, a liquid crystal display (LCD) device, an electro-luminescent display (ELD), a field emission display (FED), and a plasma display panel (PDP). At this time, in order to apply the flat panel displays in practical use, it is required to be light and have high luminance, great efficiency, high resolution, rapid response time, low driving voltage, low power consumption, low manufacturing cost and natural color display characteristics. Among the flat panel displays, the LCD device has attracted great attention by having portability and endurance as well as the aforementioned characteristics required for the flat panel displays.
The LCD device is a display device using optical anisotropy of liquid crystal. That is, when light is irradiated on the liquid crystal having polarizing characteristics according to a voltage apply state, light transmittance is controlled by an alignment state of the liquid crystal, thereby displaying a picture image. However, the LCD device requires an additional light source since the LCD device in and of itself does not emit light. One such LCD device is a reflective type LCD device. A reflective type LCD device uses ambient light but has limitations due to the environmental problems. As a result, a transmitting type LCD device having an additional light source such as a backlight has been developed. For instance, light sources such as electro-luminescence (EL), a light-emitting diode (LED), a cold cathode fluorescent lamp (CCFL) and a hot cathode fluorescent lamp (HCFL) are used for the backlight of the transmitting type LCD device. Of these, the cold cathode fluorescent lamp (CCFL) is most widely used for the backlight as the CCFL is thin and has low power consumption.
FIG. 1 is a perspective view schematically illustrating a transmitting type TN mode LCD device according to the related art. As shown in FIG. 1, the transmitting type TN mode LCD device an LCD panel having an upper substrate 11, a lower substrate 12, and a liquid crystal layer 13. At this time, the upper substrate 11 is formed of a color filter array for displaying colors, and the lower substrate 12 is formed of a thin film transistor array for selectively applying driving signals to respective pixels. Then, the liquid crystal layer 13 is formed between the upper and lower substrates 11 and 12. In addition, first and second polarizing plates 14 and 15 are formed on upper and lower surfaces of the LCD panel 10, in which polarizers of the first and second polarizing plates 14 and 15 are positioned in perpendicular to each other. Although not shown, a backlight is formed below the LCD panel 10 for irradiating light of a fluorescent lamp to the LCD panel. The backlight includes a light-guiding plate, a reflecting plate, a diffusion plate and a prism sheet.
At this time, the lower substrate 12 of the thin film transistor array includes a plurality of gate and data lines crossing each other to define a plurality of pixel regions, a plurality of pixel electrodes respectively formed in the pixel regions, and a plurality of thin film transistors at crossing points of the gate and data lines for being switched by signals of the gate lines. Also, the upper substrate 11 of the color filter array includes a black matrix layer for excluding light from portions except the pixel regions, a color filter layer for displaying R/G/B color at portions to be corresponding to the pixel electrodes, and a common electrode between the black matrix layer and the color filter layer. In this state, the pixel electrode (not shown) of the lower substrate 12 and the common electrode (not shown) of the upper substrate 11 are formed of transparent conductive metals such as Indium-Tin-Oxide ITO for transmitting the light emitted from the backlight. In case of an IPS (in-plane switching) mode LCD device, the common electrode is formed on the lower substrate.
Then, alignment layers (not shown) are formed on opposing surfaces of the upper and lower substrates 11 and 12 for aligning liquid crystal molecules 18 of the liquid crystal layer 13, whereby the liquid crystal molecules are aligned and twisted at 90° between the lower substrate 12 and the upper substrate 11. Also, the first and second polarizing plates 14 and 15 have perpendicular polarizing directions. That is, the white light emitted from the backlight is polarized to one direction according to the first polarizing plate 14, and then the polarizing light is refracted in the lower substrate 12 and the liquid crystal layer 13. At this time, as shown in the drawings, the light incident on the liquid crystal layer 13 is refracted to be in perpendicular with the direction polarized by the first polarizing plate 14 according to the liquid crystal molecules 18 rotated at 90°. Thus, it is possible to control light transmittance according to the alignment direction of the liquid crystal molecules 18 of the liquid crystal layer 13.
Next, the white light refracted by the liquid crystal layer 13 is transmitted to the upper substrate 11 having the color filter (not shown) for displaying R/G/B color, and then transmitted to the second polarizing plate 15, thereby displaying a picture image. Thus, in the general LCD device, the light transmittance is controlled by polarizing and refracting the light irradiated from the backlight, thereby displaying the picture image.
The backlight is classified into a direct type and an edge type according to the location of the fluorescent lamp. In the edge type backlight, a cylindrical fluorescent lamp is formed at one side of the LCD panel, and a transparent light-guiding plate is formed to transmit the light emitted from the fluorescent lamp to an entire surface of the LCD panel. The edge type backlight has the problem of low luminance. Also, optical design and processing technology for the light-guiding plate are required to obtain uniform luminance.
Meanwhile, the direct type backlight is suitable for a large sized LCD device of 20 inches or more, in which a plurality of fluorescent lamps are arranged in one direction below a light-diffusion plate to directly illuminate an entire surface of the LCD panel with light. That is, a direct type backlight unit having great light efficiency is commonly used for the large sized LCD device requiring high luminance. However, the direct type is problematic in that a silhouette of the fluorescent lamp may be reflected on the LCD panel. Thus, a predetermined interval has to be maintained between the fluorescent lamp and the LCD panel, so that it is hard to obtain a thin profile in the LCD device having the direct type backlight unit. As the panel becomes large, the size of the light-emitting surface of the backlight is increased. With a large-sized direct type backlight, an appropriate thickness of a light-scattering means is required. If the thickness of the light-scattering means is not appropriately thin, the light-emitting surface is not flat.
Despite this, the direct type backlight is used in a LCD device requiring high luminance and an edge type backlight unit is generally used in relatively small sized LCD devices such as monitors of laptop computers and desktop computers. With the trend towards increasingly large-sized LCD panels, the direct type backlight is actively developed by forming the plurality of fluorescent lamps under a screen, or by disposing one bent fluorescent lamp, thereby obtaining a backlight of high luminance.
FIG. 2 is a perspective view illustrating a general direct type backlight, and FIG. 3 schematically illustrates a fluorescent lamp. As shown in FIG. 2, the direct type backlight according to the related art includes a plurality of fluorescent lamps 1, an outer case 3, and a light-scattering means 5. The plurality of fluorescent lamps 1 are arranged at fixed intervals in one direction, and the outer case 3 fixes the plurality of fluorescent lamps for maintaining the fixed intervals. The light-scattering means 5 is provided above the fluorescent lamps 1. The light-scattering means 5 prevents the silhouette of the fluorescent lamps 1 from being reflected on the display surface of the LCD panel (not shown), and provides a light source with uniform luminance. For improving the light-scattering effect, the light-scattering means 5 is comprised of a plurality of diffusion sheets and one diffusion plate 5a, 5b and 5c. Also, a reflecting plate 7 is provided inside the outer case 3 for concentrating the light emitted from the fluorescent lamps 1 to the display part of the LCD panel, thereby improving light efficiency. Also, referring to FIG. 3, the fluorescent lamps 1 are respectively fixed to both sides of the outer case 3. Each fluorescent lamp 1 is a cold cathode fluorescent lamp 1, which is charged with discharge gas. Each fluorescent lamp 1 includes electrodes 2a and 2b for receiving external power (not shown), and wires 9a and 9b connected to the electrodes 2a and 2b. The wires 9a and 9b are connected to a driving circuit by an additional inverter (not shown). Thus, each fluorescent lamp 1 requires an additional inverter.
FIG. 4 is a circuit diagram schematically illustrating an inverter circuit of a backlight according to the related art. The inverter circuit according to the related art includes a DC-AC converter 31, and a plurality of output connectors 32a and 32b. At this time, the DC-AC converter 31 converts an inverter driving voltage Vcc1 to an A.C. high voltage for driving the fluorescent lamp, and then outputs the A.C. high voltage. A current flows from the plurality of output connectors 32a and 32b to both ends of the fluorescent lamp 1. The fluorescent lamp 1 is connected to the A.C. high voltage output from the DC-AC converter 31 in series.
Herein, the DC-AC converter 31 includes switching devices Q1 and Q2, and a high voltage Transformer T1. The switching devices Q1 and Q1 output a driving voltage Vcc1 to the high voltage Transformer T1 by alternately switching the driving voltage Vcc1. The high voltage Transformer 1 includes a primary coil and a secondary coil, in which the primary coil receives the driving voltage Vcc1 from the switching devices Q1 and Q2, and the secondary coil outputs a high voltage according to a winding ratio of the primary and secondary coils. Also, L1 is a line filter, R1–R3 are resistors, C1–C3 are condensers, and D1 is a diode.
Driving of the inverter circuit of the backlight according to the related art will be described as follows. For driving the inverter circuit according to the related art, the inverter driving voltage Vcc1 is input to the DC-AC converter 31 through the line filter L1, and the plurality of switching devices Q1 and Q2 of the DC-AC converter 31 alternately switches the inverter driving voltage Vcc1 by push-pull operation, thereby outputting the inverter driving voltage Vcc1 applied to a collector to the primary side of the Transformer T1. Then, the Transformer T1 outputs the voltage induced to the primary side n1 to the secondary side n2 according to the winding ratio of n1 to n2, and outputs the A.C. high voltage to the high voltage output connector 32a. 
The A.C. high voltage output from the DC-AC converter 31 is applied to a fluorescent lamp 1 through the high voltage output connector 32a and the low voltage output connector 32b. At this time, a voltage corresponding to a current flowing in the fluorescent lamp 1 and resistance value R3 is generated in the low voltage output connector 32b. Meanwhile, the backlight according to the related art includes the plurality of inverter circuits for driving the plurality of fluorescent lamps 1, and the plurality of inverter circuits are positioned at the rear of the backlight.
FIG. 5 is a circuit diagram illustrating a driving circuit provided at the rear of a backlight according to the related art. FIG. 6 is a detail view illustrating a low-voltage part of FIG. 5. As shown in FIG. 5, the backlight according to the related art further includes a high-voltage part 21, a low-voltage part 23, and a connection part 25. At this time, the high-voltage part 21 is formed at one portion of a rear side of an LCD panel 10 (not shown) to have an inverter circuit 20 (circuit of FIG. 4) converting a D.C. voltage to an A.C. voltage for driving a fluorescent lamp (1 of FIG. 2), and the low-voltage part 23 is formed at the other portion of the rear side of the LCD panel 10 to have a lower electric potential as compared to that of the high-voltage part 21. The connection part 25 is formed to connect the low-voltage part 23 to a feedback terminal (not shown) of the inverter circuit 20 of the high-voltage 21. Herein, the fluorescent lamps 1 are formed in parallel to the LCD panel 10, and power supplying lines (9a and 9b of FIG. 3) are connected to both sides of the each fluorescent lamp 1 by the high voltage output connector 32a of the high-voltage part 21 and the low voltage output connector 32b of the low-voltage part 22.
Also, the connection part 25 includes insulating wirings corresponding to the number of fluorescent lamps (1 of FIG. 2). Also, the connection part 25 includes first and second feedback connectors 22a and 22b for electrically connecting the high-voltage part 21 to the low-voltage part 23. The connection part 25 may have a signal wiring, or a plurality of wirings corresponding to the number of fluorescent lamps according to a control method of the fluorescent lamps 1. The current of the fluorescent lamps 1 is controlled according to the voltage or the current of the low-voltage part input by feedback of the inverter circuit 20. If single wiring is used, problems may occur due to different characteristics of the respective fluorescent lamps. If the plurality of wirings are used, it is possible to control the fluorescent lamps in due consideration of the impedance of the respective fluorescent lamps 1. As a result, deflection of the current is decreased among the plurality of fluorescent lamps 1, thereby providing uniform luminance by decreasing the difference of luminance among the plurality of fluorescent lamps 1.
In the backlight according to the related art, in order to connect the high-voltage part 21 to the low-voltage part 23 by using a plurality of wirings, the first and second feedback connectors 22a and 22b are connected to the respective wirings. That is, it is possible to decrease the distance between pins of the first and second feedback connectors 22a and 22b. Referring to FIG. 6, the low-voltage part of the backlight according to the related art includes a plurality of connectors 24 connected to the power supplying line 9a or 9b of each fluorescent lamp, and a second feedback connector 22b for collecting the plurality of power source lines 26 connected to the respective connectors 24, on a PCB (printed circuit board). Each connector 24 may be connected to the power supplying lines of two fluorescent lamps.
However, the backlight according to the related art has the following disadvantages.
In the backlight according to the related art, if insertion failures of the pins of the first and second feedback connectors or other failures caused by damage exist, a discharge is generated due to a voltage difference among the plurality of pins, so that the feedback connectors are burned. Thus, connectors having pins at wider intervals than the discharge distance must be used, which generates mechanical limitations on the size of the connectors.